If it's a moonstone for the north, then it'd be a sunstone for the south.

[ The moon that captures and reflects the sun's light. They've been natural companions for all of human history, really, and for good reason.

The other are harder to say. It could be water and earth, or blues and blacks for the night and day skies, or a continuation of the abstract symbolisms that the two names jewels would bring with them. ]

What's the point of hiding them behind such a complicated mess? This some sorta trial of courage? The reward better be worth it if so.